I suspect you wouldn't think that if you look into how the adaptive immune system works: A process called V(D)J recombination shuffles and rearranges the gene

I suspect you wouldn't think that if you look into how the adaptive immune system works: A process called V(D)J recombination shuffles and rearranges the genes responsible for encoding the antigen receptors on T cells and B cells. The cells are then tested for compatibility against your own tissue and cells that match your own tissue are deactivated. So new immune cells are essentially created by fuzz testing. Once deployed, immune cells that find lots of matches clone themselves amplifying their immune effect. So an autoimmune problem can be as simple as a newly minted immune cell escaping the compatibility testing or just being slightly reactive enough to your own tissue to cause problems while still inactive enough to pass the test. With that in mind its surprising to me that autoimmune disorders aren't a lot more common!